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Facies, Characterization And Early Dolomitization Of Lower Cambrian Microbialite-Dominated Reservoir,Tarim Basin,Nw China: Implications For Ultra-Deep Reservoir Quality and Distribution

Abstract

Exceptional outcrop conditions in Keping Area of northwest Tarim Basin, NW China, provide an excellent opportunity to examine the spatial distribution of sedimentary, petrophysical and multiphase dolomitization characteristics in the Lower Cambrian ultra-deep buried microbialite-dominated dolomite reservoir(average depth>6000m, up to 8000m).This study includes characterizing the sedimentary, petrophysical and multiphase dolomitization characteristics of microbialties, developing an outcrop-based reservoir model, and evaluating the contribution of original facies and early dolomitization in ultra-deep dolomite reservoir. The upper reservoir from the bottom up comprises subtidal thrombolitic boundstone and patch microbial-reef, intertidal-upper subtidal algal clastic dominated microbial influenced shoal oriented in a northeast to southwest direction over an area that encompasses about 720km2(278 mi2). These patch microbial-reef and associated thrombolitic boundstone developed abundantly in the southwest parts of the research area and attained thicknesses of 2-9m(6.6-29ft) ,but rare outcropped or developed in northern part, and are overlain by a thick section of microbially influenced algal clastic dominated packstone/grainstone with intermittent nonreservoir dolomicrite and wackstone. Porosity in the microbial dolomite reservoir includes depositional contructed void (intraframe) ,interparticle and intercrystalline, also rare diagenetic solution enhanced void and vuggy pore types. 3D pore-network reconstruction results show a strong relationship between petrophysical parameters and corresponding original facies, microbially influenced algal clastic packstone/grainstone shows mediate prososity (2.01-9.72%) with a very high connectivity of the rock pore (up to 65%), while microbial-reef and associated boundstone is characterized by high porosity(>10.05%) and very lower to mediate connectivity(average <39%). And this trend can be extended to the whole research area confidently. Early dolomitization, mainly including reflux dolomitization and seawater dolomitization, plays an important role in reservoir development and preservation, especially the latter aspect(Warren,J.,2000). Burial dolomitization will reduce reservoir porosity gradually,while hydrothermal dolomitization still need to evaluate. This work has shown that old and ultra-deep buried microbialite-dominated dolomite reservoir can still maintain high quality and widespread distribution, and provides a prediction guide using seismic.